battle-fetch: migrate to user.games.battle ConnectRPC command

The Phase 27 BattleViewer was the last UI surface still issuing raw
fetch() against the backend REST contract (`/api/v1/user/games/...
/battles/...`). The dev-deploy gateway never proxied that path, so
the viewer worked only in tools/local-dev/. Move it onto the signed
ConnectRPC channel every other authenticated surface already uses.

Wire pieces:
- FBS GameBattleRequest in pkg/schema/fbs/battle.fbs, regenerated
  Go + TS bindings.
- MessageTypeUserGamesBattle constant + GameBattleRequest struct in
  pkg/model/report/messages.go.
- pkg/transcoder/battle.go gains GameBattleRequestToPayload and
  PayloadToGameBattleRequest helpers.
- gateway games_commands.go switches on the new message type and
  GETs /api/v1/user/games/{id}/battles/{turn}/{battle_id}; the JSON
  response is re-encoded as a FlatBuffers BattleReport before being
  returned. 404 from backend surfaces as the canonical `not_found`
  gateway error.
- ui/frontend/src/api/battle-fetch.ts now builds the FBS request,
  calls GalaxyClient.executeCommand, and decodes the FBS response
  into the existing UI shape (Record<string,string> race/ship maps,
  string-form UUID). BattleFetchError carries an HTTP-style status
  derived from the result code so the active-view's not_found branch
  keeps working.
- battle.svelte pulls the GalaxyClient from the in-game shell
  context. While the layout's boot Promise.all is in flight the
  effect stays in `loading` until the client handle becomes
  non-null.
- ui/Makefile FBS_INPUTS gains battle.fbs.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

pkg/schema/fbs/battle/GameBattleRequest.go

@@ -0,0 +1,96 @@
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
+
+package battle
+
+import (
+	flatbuffers "github.com/google/flatbuffers/go"
+)
+
+type GameBattleRequest struct {
+	_tab flatbuffers.Table
+}
+
+func GetRootAsGameBattleRequest(buf []byte, offset flatbuffers.UOffsetT) *GameBattleRequest {
+	n := flatbuffers.GetUOffsetT(buf[offset:])
+	x := &GameBattleRequest{}
+	x.Init(buf, n+offset)
+	return x
+}
+
+func FinishGameBattleRequestBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.Finish(offset)
+}
+
+func GetSizePrefixedRootAsGameBattleRequest(buf []byte, offset flatbuffers.UOffsetT) *GameBattleRequest {
+	n := flatbuffers.GetUOffsetT(buf[offset+flatbuffers.SizeUint32:])
+	x := &GameBattleRequest{}
+	x.Init(buf, n+offset+flatbuffers.SizeUint32)
+	return x
+}
+
+func FinishSizePrefixedGameBattleRequestBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.FinishSizePrefixed(offset)
+}
+
+func (rcv *GameBattleRequest) Init(buf []byte, i flatbuffers.UOffsetT) {
+	rcv._tab.Bytes = buf
+	rcv._tab.Pos = i
+}
+
+func (rcv *GameBattleRequest) Table() flatbuffers.Table {
+	return rcv._tab
+}
+
+func (rcv *GameBattleRequest) GameId(obj *UUID) *UUID {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(4))
+	if o != 0 {
+		x := o + rcv._tab.Pos
+		if obj == nil {
+			obj = new(UUID)
+		}
+		obj.Init(rcv._tab.Bytes, x)
+		return obj
+	}
+	return nil
+}
+
+func (rcv *GameBattleRequest) Turn() uint32 {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		return rcv._tab.GetUint32(o + rcv._tab.Pos)
+	}
+	return 0
+}
+
+func (rcv *GameBattleRequest) MutateTurn(n uint32) bool {
+	return rcv._tab.MutateUint32Slot(6, n)
+}
+
+func (rcv *GameBattleRequest) BattleId(obj *UUID) *UUID {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
+	if o != 0 {
+		x := o + rcv._tab.Pos
+		if obj == nil {
+			obj = new(UUID)
+		}
+		obj.Init(rcv._tab.Bytes, x)
+		return obj
+	}
+	return nil
+}
+
+func GameBattleRequestStart(builder *flatbuffers.Builder) {
+	builder.StartObject(3)
+}
+func GameBattleRequestAddGameId(builder *flatbuffers.Builder, gameId flatbuffers.UOffsetT) {
+	builder.PrependStructSlot(0, flatbuffers.UOffsetT(gameId), 0)
+}
+func GameBattleRequestAddTurn(builder *flatbuffers.Builder, turn uint32) {
+	builder.PrependUint32Slot(1, turn, 0)
+}
+func GameBattleRequestAddBattleId(builder *flatbuffers.Builder, battleId flatbuffers.UOffsetT) {
+	builder.PrependStructSlot(2, flatbuffers.UOffsetT(battleId), 0)
+}
+func GameBattleRequestEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
+	return builder.EndObject()
+}